1. Field of the Invention
The present invention relates to ultrasonic transducers, and in particular, to ultrasonic transducers used as, for example, back-up sonar devices for automobiles.
2. Description of the Related Art
FIG. 15 illustrates an example of a known ultrasonic transducer. An ultrasonic transducer 1 includes a cylindrical outer case 2 with a bottom. A piezoelectric element 3 is disposed on the inner bottom surface of the outer case 2. Furthermore, a cylindrical directivity controller 4 is disposed inside the outer case 2. As shown in FIG. 16, the directivity controller 4 has thick portions and thin portions formed by, for example, a hexagonal through-hole. This directivity controller 4 is fitted into the outer case 2 so as to be in contact with the side surface and the bottom surface of the outer case 2. A back plate 5 is fitted into the outer case 2 at a position adjacent to the opening of the outer case 2, and the end portion of the outer case 2 is crimped such that the back plate 5 is fixed in place. The back plate 5 includes two terminals 6a and 6b to which lead wires 7 are connected. The terminal 6a is connected to one side of the piezoelectric element 3 via a metallic layer disposed in the back plate 5 and the outer case 2. On the other hand, the terminal 6b is connected to the other side of the piezoelectric element 3 using a lead wire 8.
This ultrasonic transducer 1 can be attached to, for example, a bumper of an automobile, and can be used as a back-up sonar device. When driving signals are input to the lead wires 7, the piezoelectric element 3 is excited, and consequently, the bottom portion of the outer case 2 is vibrated. With this configuration, ultrasonic waves are radiated along a direction orthogonal to the bottom surface of the outer case 2. The directivity of the ultrasonic waves radiated is determined by the shape of the through-hole of the directivity controller 4. When the radiated ultrasonic waves are reflected from an obstacle, the reflected waves are received by the ultrasonic transducer 1, and converted into electrical signals by the piezoelectric element 3. The electrical signals are transmitted to a receiving circuit via the lead wires 7 such that the distance to the obstacle is calculated from the time difference between the transmission and reception of the ultrasonic waves.
In this ultrasonic transducer 1, each of the outer case 2 and the directivity controller 4 can be easily processed since the outer case 2 and the directivity controller 4 are separate members. Moreover, the outer case 2 and the directivity controller 4 can be made of different materials since the outer case 2 and the directivity controller 4 are separate members. Accordingly, materials of the outer case 2 and the directivity controller 4 can be selected in accordance with desired characteristics.
Moreover, as shown in FIG. 17, an ultrasonic transducer 1 can be filled with silicon resin 9 or other suitable filler material. In this ultrasonic transducer 1, a bottom portion 2a and a side portion 2b of an outer case 2 are provided as separate members. Moreover, a back plate 5 includes a through-hole provided in a central portion thereof, and a lead wire 8 connected to a piezoelectric element 3 extends directly to the exterior via this through-hole. Foamable silicon resin 9 or other suitable material is charged inside the outer case 2 and a directivity controller 4. This silicon resin 9 absorbs unnecessary vibrations transmitted from the outer case 2. In this manner, components can be separately provided as required for their intended functions which enables the ultrasonic transducer 1 to be accurately produced at low cost (see Japanese Unexamined Patent Application Publication No. 2001-128292).
In an ultrasonic transducer, a bottom portion of an outer case is vibrated in response to the vibration of a piezoelectric element, and ultrasonic waves are radiated. When the ultrasonic waves are radiated, the vibrational energy leaks into a side portion of the outer case. When the vibrational energy leaks into the side portion of the outer case, the vibrational energy of the bottom portion is reduced, and the sound pressure of the ultrasonic waves radiated outward is reduced.
Furthermore, when a high-order spurious oscillation is generated in the outer case due to the vibrational energy that leaks into the side portion of the outer case, the oscillation of the outer case persists even when input of driving signals to the piezoelectric element is stopped. Such oscillation is referred to as reverberation. When the reverberation persists for a long period of time, the piezoelectric element continues to generate electrical signals in response to the vibration caused by the reverberation. With this, electrical signals based on the vibration of the piezoelectric element generated by the ultrasonic waves reflected from an obstacle are mixed with the electrical signals generated by the vibration caused by the reverberation, and the ultrasonic waves reflected from the obstacle cannot be detected. Such oscillation of the outer case can be absorbed to some extent using silicon resin or other suitable material charged inside the case. However, the unnecessary vibration cannot be sufficiently absorbed. In particular, when the end portion of the outer case is in direct contact with the silicon resin as shown in FIG. 17, the vibrational energy caused by the high-order spurious oscillation can be absorbed. However, since the end portion of the outer case is not in contact with the directivity controller, the leakage of the vibrational energy obtained by the piezoelectric element cannot be prevented. As a result, the sound pressure of the radiated ultrasonic waves is reduced.
Accordingly, the inventors of the present invention have tested an ultrasonic transducer including an inner case composed of a metallic material with a density higher than that of an outer case, which is disclosed in WO 2007/029559 A1. That is, a material with a high density has a high acoustic impedance, and such a material is not easily vibrated. Therefore, when the inner case is made of a material with a high acoustic impedance, the leakage of the vibrational energy into the side portion of the outer case is reduced, and the sound pressure of the radiated ultrasonic waves is increased. However, reverberation is still generated at the end portion of the outer case when the end portion of the outer case is not in contact with the inner case.